Quality Water for Idaho Current Information Series No. 962

Over 90 percent of the drinking water consumed in Idaho is supplied by groundwater. Because this resource is so vital to Idahoans, best management practices (BMPs) for agricultural management are becoming more important. Nitrate is the most common groundwater pollutant in Idaho and in the United States. Nitrates in groundwater can originate from many sources, including agriculture, septic tanks, landfills, lawns and gardens, industry and municipalities.

Nitrogen is an element essential for all plant and animal life. The interlocking succession of nitrogen reactions occurring in the soil is known as the nitrogen cycle (fig. 1). Agriculture affects both nitrogen additions and subtractions to the soil. Additions include nitrogen fertilizers, crop residues, nitrogen fixation by legumes, and manures. Subtractions attributed to agriculture include crop removal (harvesting), plant uptake, and nitrogen leaching.

Fig. 1. The nitrogen cycle.

Federal and state standards dictate that drinking water should not contain more than 10 parts per million (ppm) NO₃-N. In rural areas of Idaho, potentially significant sources of nitrogen for groundwater contamination include nitrogen fertilizers, private septic systems, livestock feedlots, barnyards, and legumes used as green manures.

Specific types of BMPs for nitrogen fertilizer management that should be employed in many areas of Idaho include:

1. Soil sampling
2. Fertilizer recommendations based on research
3. Timing of fertilizer application
4. Fertilizer placement
5. Nutrient credits for legumes and manures
6. Nitrification inhibitors
7. Manure management
8. Irrigation systems management
9. Slow-release nitrogen fertilizers
10. Crop rotation selection
11. Variable fertilizer management

The need frequency of soil tests for a nutrient depends on such things as its mobility in the soil and the nutrient requirement of the crop to be grown. Soil samples for determination of phosphorus, potassium, and micronutrients should be taken at least once during each crop rotation cycle. For best soil fertility management, especially for mobile nutrients such as nitrogen and sulfur, soil testing should be done each year, and crops should be fertilized for a realistic crop yield goal. Having an analysis performed for every nutrient each year is not necessary. A record of soil test results should be maintained on each field to evaluate long-term trends of nutrient levels.

The University of Idaho has developed more than 30 fertilizer guides for Idaho crops. The data base used to develop these fertilizer guides is extensive and has been collected for over three decades. Fertilizer guides take into account the amount of residual nitrogen in the soil profile, the amount of nitrogen mineralized (released) from organic matter decomposition during the growing season, crop yield potential, and plant residue from the previous crop.

Some BMPs for timing of fertilizer applications include: (1) applying nitrogen to a cool season crop in the spring instead of the previous fall, (2) applying only a portion of the needed nitrogen as a preplant treatment, (3) using split or multiple nitrogen applications where appropriate, (4) using side-dressed or top-dressed applications during the growing season if irrigated or adequate precipitation is expected to move it into the root zone, and (5) using a combination of tissue analysis for diagnosis and topdressed nitrogen applications during the growing season.

Sound nitrogen management alone will not prevent groundwater contamination under irrigated conditions. Even with the most accurate nitrogen application, overirrigation can cause nitrate to move below the crop rooting zone. Consequently, best management practices for proper irrigation scheduling need to be included. A sound irrigation management program considers soil water-holding capacity, crop growth stage and anticipated water use, evaporation rate, rainfall, and previous irrigation to determine the timing and amount of irrigation water to be applied.

• Use preplant soil profile nitrate testing and soil and plant nitrate testing when appropriate during the growin season.
• Base nitrogen application rates on realistic yield goals.
• Credit nitrogen contributions from legumes, manures, and other organic wastes.
• Plan nitrogen applications to correspond with crop demand and availability to the crop.
• Do not apply nitrogen fertilizer in the fall on coarse textured soils, on shallow soil over fractured bedrock, or on soils with a water table close to the soil surface.
• Use nitrification inhibitors when soil conditions and nitrogen application timing may promote leaching.
• Uniformly apply manure across a field in accordance with crop nutrient requirements.
• Schedule irrigation to minimize leaching.
• Manage fertigation systems carefully.
• Diversify crop rotations to include crops that utilize deep residual nitrogen.

This publication is one of a series on water quality issues produced by the University of Idaho Cooperative Extension System for the people of Idaho. The material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 90-EWQUI-1-9216.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

10,000, November 1992 Printed with special grant funds from USDA

Comments to author: karenl@uidaho.edu

All contents copyright © 1997-2002.
College of Agricultural and Life Sciences, University of Idaho.
All rights reserved.

Revised: January 3, 2002

URL: http://www.uidaho.edu/wq/wqpubs/cis962.html